The present invention relates to the general field of gas turbines, and more particularly to controlling a starter/generator.
The field of application of the invention is that of gas turbines for the engines of airplanes or helicopters, and also for auxiliary power units (APUs).
In a gas turbine, certain pieces of equipment or accessories are driven by a mechanical transmission with mechanical power taken from a turbine shaft. This mechanical transmission, referred to as an accessory gearbox (AGB) comprises a set of gearwheels housed in a casing and mechanically coupled to accessories. The accessories include in particular various pumps for producing hydraulic energy, for feeding fuel, and for lubrication, and also in certain applications one or more electrical starter/generators (S/Gs). The speed of the S/G as determined by the AGB is in particular proportional to the speed of operation of the engine, with the coefficient of proportionality corresponding to the gear ratio of the gear system in the AGB.
When the gas turbine is in operation, the or each S/G operates as an electricity generator and produces a supply voltage that powers one or more electrical energy distribution centers for the aircraft and for its engine(s).
When the gas turbine is not operating or is operating at a slow speed of rotation, an S/G may operate as a starter by being powered by an external source of energy in order to set the gas turbine into operation by rotating the turbine shaft to which the AGB is connected.
Optimal mechanical operating conditions for an S/G are contradictory depending on whether it is operating as a starter or as a generator.
For given mechanical power, when an S/G is operating as a starter, it is desired to give priority to high speeds of rotation so as to minimize the torque on the S/G, while ensuring sufficient torque to enable the engine to start. Without any change in gear ratio, using a ratio that gives priority to high speeds in starter mode gives rise to a large range of speed variation in generator mode, thereby increasing the range of generator frequencies in a manner that is harmful to integrating the S/G in the network of the aircraft. In particular, the upper limit may be unacceptable (greater than the 800 hertz (Hz) that is generally acceptable on certain commercial airplanes).
The gear ratio between the turbine shaft and the S/G is consequently selected so as to provide an acceptable compromise between S/G operation as a starter and as a generator. In certain commercial airplanes, generator mode determines the gear ratio to the detriment of starter mode.